Reduce QtWidgets dependencies in the Cocoa plugin.

Reduce QtWidgets dependencies in the Cocoa plugin.

b8246f08 added the new menus implementation which
removes one of the major QtWidget dependencies.

Continuing on the path to QtWidget-freedom this
commit removes the following:
- qmenu_mac.h/mm. Not used.
- FORMS += filedialog.ui. Not used.
- qt_strippedText from qaction.cpp. Duplicate.
- Misc stray includes.

What is left should be the dependencies needed by
the print engine implementation. These will be
handled at a later time.

Change-Id: I0cead4cdaddcebd402d30c6112f3e889d0f28740
Conflicts:
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Bradley T. Hughes <bradley.hughes@nokia.com>